1. Field of the Invention
The present invention relates to a gas sensor element for detecting concentration of a specific gas within a measured gas, and a gas sensor including the gas sensor element.
2. Description of the Related Art
Gas sensors are widely used for detecting leakage of combustible gas serving as fuel and detecting gas included in exhaust gas. To measure oxygen gas concentration within exhaust gas, solid electrolyte sensors that detect electromotive force attributed to a specific gas and in which a battery is composed of ionic conductors have been used since the past.
For example, Japanese Unexamined Patent Publication H08-8044 discloses a gas sensor element included in a gas sensor shown in FIG. 1. As shown in FIG. 1, a gas sensor element 91 includes a solid electrolyte body 97 having oxygen ion conductivity, a wiring layer 92 formed on both main surfaces of the solid electrolyte body 97, and an insulating layer 93 layered on one main surface 970 of the solid electrolyte body 97.
An electrode terminal 94 for connecting to an external device is formed on a surface of the insulating layer 93. An intermediate layer 95 is interposed between the electrode terminal 94 and the wiring layer 92. A through hole 96 is formed in the insulating layer 93, and a metal connecting member 96a is provided within the through hole 96. The electrode terminal 94 and the wiring layer 92 are electrically connected by the connecting member 96a and the intermediate layer 95.
The gas sensor element 91 includes a heater section 980 for heating the solid electrolyte body 97. The heater section 980 includes the wiring layer 92, the insulating layer 93, the heater connecting member 911, the intermediate layer 95, the connecting member 96a, and the electrode terminal 94. A heater connecting member 911 is interposed between the wiring layer 92 and the intermediate layer 95. Furthermore, a heating element (not shown) is connected to the wiring layer 92.
The gas sensor element 91 is used in a state heated by the heater section 980. Therefore, as a result of the overall section (the wiring layer 92, the intermediate layer 95, the connecting member 96a, the electrode terminal 94, and the heater connecting member 911) being composed of a metal material of which the main component is platinum, the gas sensor element 91 is capable of withstanding high temperature environments.
However, because the heater section 980 in the conventional gas sensor element 91 is composed of a metal material of which the main component is platinum, manufacturing cost is high. Therefore, a gas sensor element 91 capable of being manufactured at a low cost is desired.
As shown in FIG. 2, an attempt has been made to reduce manufacturing cost of the gas sensor element 91 by forming the wiring layer 92 from palladium that is less expensive than platinum (Pt). When the gas sensor element 91 is manufactured, as shown in FIG. 2, the insulating layer 93, the wiring layer 92, the solid electrolyte body 97, and the like are stacked and subsequently fired. Because the wiring layer 92 (palladium) and the intermediate layer 95 (platinum) are composed of differing materials, when fired, platinum and palladium alloy is formed on an interface 99 between the wiring layer 92 and the intermediate layer 95. In accompaniment with the alloying, metal in the periphery moves towards the interface 99. Because palladium has a lower melting point than platinum, when fired, palladium moves to the interface 99 before platinum as shown in FIG. 3. As a result, a void 90 is formed in the wiring layer 92, causing disconnection in the wiring layer 92 in some instances. Electrical resistance between the electrode terminal 94 and the wiring layer 92 becomes high. Therefore, temperature rise in the gas sensor element 91 becomes insufficient and electrical resistance in the gas sensor increases. As a result, detection accuracy of the gas sensor element 91 may decrease. Alternatively, the gas sensor element 91 may become unable to perform detection.
Furthermore, the electrode terminal 94 and the wiring layer 92 are used within a wide temperature range, such as from −40° C. to 1000° C. Therefore, when the electrode terminal 94 and the wiring layer 92 are formed using metal materials having differing coefficients of thermal expansion, stress occurs as a result of the difference in thermal expansion, and disconnection may occur in the wiring layer 92. Electrical resistance between the electrode terminal 94 and the wiring layer 92 becomes high. As a result, as described above, detection accuracy of the gas sensor element 91 may decrease. Alternatively, the gas sensor element 91 may become unable to perform detection.